Abstract

Adjusting pH with an ammonia solution during the synthesis, single-crystalline BiVO4 has been prepared using Bi(NO3)3·5H2O and NH4VO3 as starting materials through aqueous-phase precipitation at room temperature. The prepared samples are characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM). The impact of pH on structure, surface morphology, visible-light photocatalytic activity, and light absorption performance of BiVO4 is explored and discussed. During the synthesis process, neither extremely acidic (low pH) nor basic (high pH) conditions are desirable for the formation of BiVO4 in monoclinic phase. The highest photocatalytic performance on the degradation of a methylene blue solution is observed under pH=7.0 for BiVO4 in monoclinic scheelite, which is attributed to its small grain size and marked surface oxygen evolution ability.

1. Introduction

Titanium dioxide and other semiconductor photocatalysts have been intensively studied for wide potential applications in water and air purification and solar energy conversion since Fujishima and Honda first reported the photocatalytic splitting of water on TiO2 electrodes in 1972 [1–9]. Among the various oxide and nonoxide semiconductor photocatalysts, titanium dioxide has proven to be the most suitable for widespread environmental applications due to its chemical and biological inertness, very strong oxidizing properties, low cost-effectiveness, and long-term stability against photocorrosion and chemical corrosion [7, 8]. However, a large intrinsic band gap of TiO2 (3.2 eV for anatase and 3.0 eV for rutile) allows only a small portion of the solar spectrum in the ultraviolet light region to be absorbed. Therefore, effective utilization of visible light has become one of the most difficult challenges in photocatalysis, and it is highly desirable to develop a photocatalyst that can use visible light in high efficiency under sunlight irradiation [6–9].

Bismuth vanadate (BiVO4) has attracted more and more attention as visible-light-response photocatalyst [10–16]. According to previous reports, BiVO4 appears in three main crystalline phases: monoclinic scheelite (s-m), tetragonal zircon (z-t), and tetragonal scheelite (s-t) [17–19]. Among these forms, BiVO4 (s-m) presents strong visible-light catalytic properties [14–16], because the energy gap of BiVO4 (s-m) is merely 2.41 eV, and the absorptive wavelength is 515 nm [14]. Therefore, a wide range of exciting light results in high catalytic activity. It is well known that the catalytic activity is influenced by many factors such as structure, crystallite dimension, and surface appearance [20–23]. Moreover, the activity is strongly connected to the preparation method and conditions of the catalyst.

BiVO4 (s-m) is usually prepared through solid-state reaction at high temperature [24, 25]. Hydrothermal synthesis [13, 26–28], organometallic decomposition [29], and aqueous-phase precipitation [11, 12, 30, 31] have also been reported to obtain BiVO4 (s-m). As discussed by Kudo and coworkers [11], it took a long time to prepare BiVO4 with different structures through aqueous-phase precipitation using particular starting materials (e.g., K2V5O and KV3O8) at room temperature. Utilizing Bi(NO3)3 and NH4VO3 as starting materials, Yu and coworkers obtained BiVO4 (s-m) through calcination at 250°C [12], and they studied the correlation between the calcination temperature and crystalline phase in an ammonia solution. Furthermore, using the same starting materials, Ke and coworkers prepared BiVO4 (s-m) through aqueous-phase precipitation mixed with carbamide at 364°C [30]. Utilizing NaVO3 as the vanadium source, Gao and coworkers successfully prepared BiVO4 (s-m) through aqueous-phase precipitation at 150°C [31].

BiVO4 powders have also been synthesized through different templating routes. For example, Li and coworkers synthesized BiVO4 (s-m) using mesoporous silica KIT-6 as a hard template [32]. Ke and coworkers selectively prepared BiVO4 through a hydrothermal process using cetyltrimethyl-ammonium bromide as a template-directing agent [27]. However, the template-assisted route not only increases the cost but also makes it more difficult to scale up due to its complexity and the limited capability of a template. To our best knowledge, little has been reported on the synthesis of BiVO4 (s-m) with various morphologies through regulating pH in convenient aqueous-phase precipitation, along with its visible-light photocatalytic activity. In this paper, utilizing aqueous-phase precipitation with Bi(NO3)3·5H2O and NH4VO3 as starting materials, single-crystalline BiVO4 (s-m) is successfully prepared. The impact of pH on structure, surface topography, visible-light photocatalytic activity, and light absorption performance of BiVO4 (s-m) is investigated, and the results are discussed.

2. Experimental

2.1. Preparation of BiVO4

Equimolar amounts of NH4VO3 and Bi(NO3)3·5H2O were dissolved consecutively into a nitric acid solution, and the mixture was kept stirring for 30 min. To the mixture was slowly added an NH4VO3 solution to give a uniform and transparent saffron yellow solution. An orange precipitate was obtained after the pH was adjusted by dropwise adding an ammonia solution under stirring. The precipitate was then filtered, rinsed with distilled water, and dried at 25, 80, or 110°C to give a final product.

2.2. Characterization

X-ray diffraction (XRD) patterns were measured on an X-ray diffractometer (Rigaku dmax/III-C) with Cu Ka radiation (tube voltage = 35 KV, tube current = 15 mA) in order to analyze the structure of BiVO4. DRS patterns were obtained to appraise the light absorption performance of samples. Scanning electron microscope (SEM, HITACHI S-3500N) was used to observe the micromorphology. Absorbance for degradation of a methylene blue (MB) solution was measured on a UV-vis spectrophotometer (U-3010, Hitachi). X-ray photoelectron spectroscopy (XPS) patterns were measured using X-ray photoelectron spectroscopy (ESCALAB MK II) to analyze the surface components with Al Kα radiation, and all spectra were referenced by setting the hydrocarbon C1s peak to 285.0 eV to compensate for residual charging effects.

2.3. Measurements of Photocatalytic Activity

Photocatalytic decomposition of a methylene blue (MB) solution was used to evaluate the photocatalytic properties of the synthesized materials. A photocatalyst powder (0.10 g) was dispersed in an MB solution (100 mL) in a reaction cell with a magnetic stirrer. A 300 W Xe-illuminator (light intensity = 3.5 mW/cm2) was used as a light source, and a cutoff filter (λ > 420 nm) was employed for visible-light irradiation in order to decolorate. At irradiation time intervals of 1 h, suspensions (4.0 mL) were collected and subsequently centrifuged to remove photocatalyst particles. A spectrophotometer was used to analyze the absorbance. Destruction rates were calculated with formula: 𝐷=(𝐶0−𝐶)/𝐶0×100% (𝐷: destruction rate; 𝐶0: initial concentration; 𝐶: concentration in different batches).

3. Results and Discussion

3.1. Formation of BiVO4 with Different Crystal Phase

Although similar to the XRD pattern of BiVO4 (s-t) due to the same scheelite structures, the XRD pattern of BiVO4 (s-m) exhibits a weak diffraction peak at 15°. There are apparent splitting peaks at 2𝜃=18.5, 35, and 46°, which are attributed to the Bi-O polyhedron in BiVO4 (s-m) containing a 6s2 lone pair in Bi3+ [12].

In general, BiVO4 (s-m) and BiVO4 (s-t) are obtained at high temperatures. In this work, two different single-crystalline BiVO4 (s-m) and BiVO4 (s-t) are successfully prepared at low temperature. Moreover, the photocatalytic activity of BiVO4 (s-m) is superior to that of BiVO4 (s-t). To optimize the process conditions for BiVO4 (s-m), the impact of different pH on the formation of crystalline BiVO4 at 110°C is studied as shown in Figure 2.

3.2. Impact of Preparative pH on the Formation of BiVO4

Figure 2 shows the XRD patterns of the samples prepared under different pH at 110°C. The peaks of the powders under pH = 1.0 and 10.0 can both be indexed to mixtures of BiVO4 (z-t) and BiVO4 (s-m) (JCPDS no. 14-0133 and no. 14-0688), while the samples under pH = 3.0, 5.0, and 7.0 are BiVO4 (s-m) only. Therefore, BiVO4 (s-m) can be selectively synthesized simply by adjusting pH. During the synthetic process, neither extreme acidic (low pH) nor basic (high pH) conditions are desirable to the formation of monoclinic phase.

The average particle size of the samples is calculated using the Scherrer equation [6],
𝐷=𝐾(𝜆×(180∘/𝜋))√𝛽2−𝑆2cos𝜃𝛽,(1)

where 𝐷 is the apparent crystallite mean size, perpendicular to the reflecting plane, 𝐾 the shape factor (0.9 for spherical crystallite), 𝜆 the radiation wavelength (1.5406 A°), 𝑆 the instrumental line broadening (0.001°), 𝛽 the reflection width at half-maximum intensity (FWHM), and 𝜃𝛽 is the angle at the maximum intensity. The results are summarized in Table 1. Under extreme acidic (low pH, pH = 1.0) or basic (high pH, pH = 10.0) conditions, only BiVO4 (z-t) is formed with a large particle size. The sample grain obtained in the presence of ammonia decreases gradually with an increasing amount of ammonia, and in the end it transforms into BiVO4 (s-m).

Table 1: Crystal size of samples different pH at 110°C.

According to Tokunaga’s et al. view [14], although BiVO4 (s-m) is thermodynamically more stable than BiVO4 (z-t) at room temperature, the formation of BiVO4 (z-t) seems to be favored kinetically with a sudden increase in pH by adding a base. BiVO4 (z-t) with a large particle size shows no phase transform into the thermodynamically stable BiVO4 (s-m) at room temperature. The particle size becomes smaller during the preparation after dissolution and recrystallization, and BiVO4 (s-m) is finally formed after recrystallization and/or in the small particles, where the stress can be released.

3.3. Impact of Preparative pH on the Morphology of BiVO4

Figure 3 shows SEM micrographs of BiVO4 under different preparative pH at 110°C, with significant differences found in the morphology and particle shape. When the pH of the solution is approximately neutral, the particle size of the samples is 19.4 nm, while when the condition is extremely acidic or basic, the particle size becomes relatively larger. The results are consistent with Table 1. Obviously, the pH of the synthesis solution has a significant impact on the morphologies and particle size of the final products.

3.4. Impact of Preparative pH on the Optical Absorption Performance of BiVO4

Figure 4 presents the DRS spectra of BiVO4 prepared under different pH at 110°C. Although there are some differences in the absorption band of BiVO4 under different preparative pH, all absorption bands are found in the visible range. Samples prepared under extremely acidic (pH = 1.0) or basic (pH = 10.0) conditions exhibit blue-shift in the spectra when compared with those under pH = 3.0, 5.0, and 7.0, and the optical absorption performance of the former is weaker than that of the latter. The absorption band in the visible-light region is attributed to the band transition from a Bi6s and O2p valence band to a V3d conduction band [29]. If an absorption band is different, both the relevant conduction and valence band energy level are also difference. The band gap (𝐸𝑔, eV) is calculated using the equation 𝐸𝑔 = 1240/λ, where λ is the wavelength of the absorption bands. The band gap of BiVO4 (z-t) is slightly wider than that of BiVO4 (s-m). If the BiVO4 (s-m) content is low and its band gap is relatively wide, the absorption band will be blue-shifted. According to the DRS spectra, low BiVO4 (s-m) contents are obtained under extremely acidic (pH = 1.0) or basic (pH = 10.0) conditions, while under pH = 3.0, 5.0, and 7.0, only BiVO4 (s-m) is formed. This is consistent with the XRD results. The low optical absorption performance of the samples under extremely acidic (pH = 1.0) or basic (pH = 10.0) conditions may be attributed to their weak crystallinity.

3.5. Impact of Preparative pH on the Photocatalytic Activity of BiVO4

The photocatalytic activities of BiVO4 are measured on the degradation of an MB solution in a liquid medium under a visible-light irradiation. Figure 5 presents the MB concentration (𝐶/𝐶0) in a function of the irradiation time over samples under different preparative pH at 110°C. As shown in Figure 5, the photocatalytic activity of BiVO4 (s-m) is generally higher than the mixtures of BiVO4 (z-t) and BiVO4 (s-m). Especially, the decoloration rate of MB over the sample under pH = 7.0 is 98.9% after 3 h of irradiation, and the sample also exhibits the highest photocatalytic activity. The decoloration rates are 91.2% and 84.0% over the samples under pH = 5.0 and 3.0, respectively. However, the photocatalytic activities of the samples under pH = 1.0 and 10.0 are both low, whose decoloration rates are 38.0% and 30.0%, respectively.

It is widely believed that the photocatalytic activity is directly affected by the particle size of a catalyst. The smaller the particle size is, the larger the specific surface area becomes. The different particle size in Table 1 also explains the order for the photocatalytic activity of the samples under different pH. The average grain size of the sample under pH = 7.0 is the smallest, which possesses the highest photocatalytic activity.

3.6. X-Ray Photoelectron Spectroscopy Analysis

In order to explore the surface adsorption of OH− on the catalyst surface, XPS analysis was performed. The XPS spectra of BiVO4 (s-m) under different pH are shown in Figure 6. The three fitting peaks of O1s are from lattice oxygen (VO), adsorbed oxygen (OH, binding energy: 532 eV), and carbon oxygen (CO, binding energy: 553 eV). A C–O bond is formed from carbon impurities in the air and adsorbed oxygen on the surface of catalyst. The percentages of the fitting peak area on the lattice oxygen (VO), adsorbed oxygen (OH), and carbon oxygen (CO) are summarized in Table 2.

Table 2: Binding energy and area percentage of BiVO4 (s-m) for the O1s region under different pH.

Table 2 summaries the results of the curve-fitting of the high resolution XPS spectra of BiVO4 (s-m) for the O1s region under different pH. With an increasing pH, the relative content of adsorbed oxygen increases gradually. Photocatalytic reactions are usually caused by photoelectrons and holes separated, which can provide highly active OH• [33]. A strong oxide free radical, OH•, has no selectivity toward oxidizing certain organic compounds. Therefore, the better separation between photoelectron and hole attribute to the higher surface adsorbed oxygen contents. Sequentially, the quantum efficiency of the catalyst is enhanced. Usually, adsorbed oxygen and carbon oxygen reflect the surface adsorption ability of a photocatalyst. The surface-adsorbed oxygen content of the sample under pH = 7.0 is the highest (25.57%+17.61%), so it exhibits a high photocatalytic activity.

4. Conclusions

Single-crystalline BiVO4 (s-m) has been synthesized through aqueous-phase precipitation at pH ranging from 3.0 to 7.0 regulated with an ammonia solution. The morphology, surface texture, and grain size of the synthesized BiVO4 are significantly dependent on the preparative pH. Under pH = 1.0 and 10.0 conditions, mixtures of BiVO4 (s-t) and BiVO4 (s-m) are obtained, while under pH = 3.0–7.0, only BiVO4 (s-m) is prepared. Therefore, BiVO4 (s-m) can be selectively synthesized simply by adjusting the preparative pH. The band gap of BiVO4 (s-m) is 2.4 eV when it is prepared under pH = 5.0 and 7.0.

The overlap between the Bi6s and O2p orbitals in the valence band results in an increase in hole mobility. Both the surface oxygen evolution ability and grain size have significant impacts on the photocatalytic performance for the degradation of a methylene blue solution under a visible-light irradiation.

Acknowledgment

This work is supported by the National Natural Science Foundation of China (NSFC) (Grant no. 20563004).